CMOS image sensor and method for fabricating the same

ABSTRACT

A CMOS image sensor that includes a semiconductor substrate with a plurality of photodiodes arranged at fixed intervals on the semiconductor substrate. A light-shielding layer partially overlapping the plurality of photodiodes and an insulating interlayer are formed on an entire surface of the semiconductor substrate including the plurality of photodiodes. A color filter layer having a plurality of color filters separated by a predetermined gap is formed on the insulating interlayer and a planarization layer is formed over the entire surface of the semiconductor substrate including the color filter layer. A plurality of microlenses are formed on the planarization layer in correspondence with the color filters of the color filter layer, wherein an additional structural layer, disposed between the color filter layer and the insulating interlayer, is provided to close a predetermined gap between the color filters of the color filter layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/312,352,filed Dec. 21, 2005, and claims priority to Korean Patent ApplicationNo. 10-2004-0112056, filed on Dec. 24, 2004, both of which are herebyincorporated by reference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image sensor, and more particularly,to a CMOS image sensor and a method for fabricating the same, whichimproves the sensor's image characteristics by employing an additionalstructural layer for closing a gap between elements of a color filterarray.

2. Discussion of the Related Art

Image sensors are semiconductor devices that convert an optical imageinto an electrical signal and include charge-coupled devices andcomplementary metal-oxide-semiconductor (CMOS) image sensors. A typicalcharge-coupled device includes an array of photodiodes converting lightsignals into electrical signals. This set up, however, has a complicateddriving method, high power consumption, and a complicated fabricationprocess requiring a multi-phased photolithography process. Additionally,in a charge-coupled device, the integration of complementary circuitrysuch as a control circuit, a signal processor, and an analog-to-digitalconverter into a single-chip device is difficult, thereby hinderingdevelopment of compact-sized (thin) products, such as digital stillcameras and digital video cameras, that use such image sensors.

CMOS image sensors, on the other hand, adopt CMOS technology that uses acontrol circuit and a signal processing circuit as a peripheral circuitand adopts switching technology which allows outputs to be sequentiallydetected using MOS transistors provided in correspondence with thenumber of arrayed pixels. Additionally, a CMOS image sensor uses CMOSfabrication technology, which is a simple fabrication method with fewerphotolithography steps, that enables an advantageous device exhibitinglow power consumption.

Typically, in the aforementioned CMOS image sensor, a photodiode is theactive device that forms an optical image based on incident lightsignals by generating electrical signals according to the intensity andwavelength (color) of the incident light. In such a CMOS image sensor,wherein each photodiode senses incident light and a corresponding CMOSlogic circuit converts the sensed light into an electrical signalaccording to the input wavelength, the photodiode's photosensitivityincreases as more light is able to reach the photodiode. One way ofenhancing a CMOS image sensor's photosensitivity is to improve its “fillfactor,” i.e., the degree of surface area covered by the photodiodesversus the entire surface area of the image sensor. The fill factor isimproved by increasing the area responsive to incident light. Theconcentration of incident light onto the photodiode is furtherfacilitated when the quantum efficiency at all wavelengths (white light)is “1,” which represents a balanced transmission to the photodiodesacross the spectrum to include complimentary components of red light,blue light, and green light received by the photodiodes.

To redirect any light that may be incident to the image sensor outsidethe immediate area of the photodiodes and to concentrate (focus) theincident light on one or more of the photodiodes themselves, a device ofa material exhibiting excellent light transmittance, such as a convexmicrolens having a predetermined curvature for refracting incidentlight, may be provided. Incident light striking the surface of theconvex structure of the microlens while in parallel to the optical axisof the microlens is refracted by the microlens according to thecurvature of the convex microlens and is thereby focused at apredetermined point along the optical axis. As FIG. 1 illustrates, aCMOS color image sensor according to the related art may be providedwith a microlens layer over a color filter layer including red (R), blue(B), and green (G) filter elements for focusing the light of each color(wavelength).

Referring to FIG. 1, a CMOS image sensor according to the related artincludes a semiconductor substrate 100, on which a plurality (array) ofphotodiodes 110 are arranged at fixed intervals; a light-shielding layer120 for allowing a light signal reception at each photodiode only byblocking any light arriving between the photodiodes; an insulatinginterlayer 130 formed over the structure of the light-shielding layerand photodiode array to receive a color filter layer 150 which ispatterned into a color filter array including R, G, and B elementscorresponding to the photodiode array; and a planarization layer 160formed over the entire resulting structure to receive a plurality ofmicrolenses 170 corresponding to the color filter array. The individualcolor filters of the color filter layer 150, which are arranged at fixedintervals, are formed by coating and patterning a correspondingphotosensitive material on the insulating interlayer 130. Each coatingis patterned by a photolithography step using a separate mask to form acorresponding array of pixels. Each element of the color filter layer150 is arranged to transmit light of a predetermined wavelengthcorresponding to the element color.

A predetermined gap inherently occurs along the junctures betweenadjacently arranged filters that are meant to filter different colors.While the size of this gap is preferably kept to a minimum, itsexistence is due to the unavoidable error tolerances of thephotolithography process used to form the color filter layer during thepatterning process of the respective colored resists that make up thecolor filter layer. The presence of the gap, which is especiallyproblematic at the junction of four adjacently disposed filters arrangedas part of the Bayer pattern of a color filter array, precludes optimumphotodiode operation because the incident light allowed to passunfiltered by the color filter layer degrades the image characteristics.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a CMOS image sensorand a method for fabricating the same that substantially obviates one ormore problems due to limitations and disadvantages of the related art.

One advantage of the present invention is that it can provide a CMOSimage sensor, which improves image characteristics by closing the gapbetween the elements of a color filter array.

Another advantage of the present invention is that it can provide a CMOSimage sensor with improved image characteristics due to the blocking oflight incident to a gap existing between the elements of a color filterarray.

Another advantage of the present invention is that it can provide a CMOSimage sensor having a color filter array with improved imagecharacteristics obtained by including an additional structural layer inconjunction with the color filter layer.

Another advantage of the present invention is that it can provide a CMOSimage sensor that prevents degradation of the image-generatingcharacteristics of a photodiode array of the image sensor due to gapsexisting between elements of a corresponding color filter array.

Another advantage of the present invention is that it can provide a CMOSimage sensor having a color filter array with improved imagecharacteristics by providing greater color separation (delineation)while approximating a color filter array having no gaps betweenindividual color elements.

Another advantage of the present invention is that it can provide amethod for fabricating a CMOS image sensor with any of the abovementioned advantages.

Additional examples of advantages and features of the invention will beset forth in part in the description which follows, and in part willbecome apparent from the description, or by practice of the invention.

To achieve these objects and other advantages in accordance with anembodiment of the present invention, as embodied and broadly describedherein, there is provided a CMOS image sensor, comprising a color filterlayer having a plurality of color filters separated by a predeterminedgap; and an additional structural layer having a pattern for blockinglight incident to the color filter layer at the predetermined gap.

In another aspect of the present invention, there is provided a methodfor fabricating a CMOS image sensor comprising forming, in asemiconductor substrate, a plurality of photodiodes at fixed intervals;forming an insulating interlayer on an entire surface of thesemiconductor substrate including the plurality of photodiodes; forming,on the insulating interlayer, a color filter layer having a plurality ofcolor filters separated by a predetermined gap; forming an additionalstructural layer on the insulating interlayer; forming a planarizationlayer on the entire surface of the semiconductor substrate including thecolor filter layer; and forming a plurality of microlenses on theplanarization layer in correspondence with the color filters of thecolor filter layer, wherein the additional structural layer is disposedbetween the color filter layer and the insulating interlayer and ispatterned to close the predetermined gap between the color filters ofthe color filter layer.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a cross-sectional view of a CMOS image sensor according to therelated art; and

FIGS. 2A-2D are cross-sectional views of the process for fabricating aCMOS image sensor according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, like reference designations will be usedthroughout the drawings to refer to the same or similar parts.

FIGS. 2A-2D illustrate a process for fabricating the CMOS image sensoraccording to the present invention.

As shown in FIG. 2A, a plurality of photodiodes 210 are formed at fixedintervals on the surface of a semiconductor substrate 200. In lieu ofphotodiodes, photosensitive gates of a transistor (not shown) may serveas the active device for sensing an incident light signal received via acolor filter array to generate an image by outputting an electricalsignal based on the light signal. A layer of opaque metal, for examplechromium, is formed on the entire surface of the semiconductor substrate200 including the photodiodes 210. The chromium layer is selectivelypatterned by photolithography leaving the opaque metal on portions ofthe semiconductor substrate 200 exposed between the photodiodes 210 andcovering the exposed portions completely, thereby forming alight-shielding layer 220 for allowing a light signal reception at eachphotodiode only by blocking any light arriving between the photodiodes.An insulating interlayer 230, which may be formed as a multi-layeredstructure, is formed on the entire surface of the semiconductorsubstrate 200 including the light-shielding layer 220.

As shown in FIG. 2B, a black photoresist is coated on the insulatinginterlayer 230 and it is then patterned by an exposure and developmentprocess (i.e., photolithography) to form an additional structural layer240 for blocking any light incident to a predetermined gap that willoccur between elements of a color filter array to be formed on theinsulating interlayer 230. The additional structural layer 240 may havea pattern analogous to that of the light-shielding layer 220. Therespective widths of individual elements of the pattern of theadditional structural layer 240 are preferably greater than thecorresponding widths of the gaps that inherently occur along thejunctures between adjacently arranged filters.

Referring to FIG. 2C, layers of colored resists for filtering light bywavelength are successively coated over the entire surface of thesemiconductor substrate 200 including the additional structural layer240. Each coating is patterned in turn by a photolithography step togive a color filter layer 250 in which the perimeters of the individualelements (i.e., the red, green, and blue filters) of the color filterlayer partially overlap adjacent portions of the additional structurallayer 240. The additional structural layer 240 is thereby arrangedbetween pattern elements of the color filter layer 250 and issubstantially disposed between the color filter layer and the insulatinginterlayer 230 structurally provided under the color filter layer. Theoverlap ensures that any light incident to the inter-element gaps iscompletely blocked and will not enter the insulating interlayer 230 orreach the photodiode array without being filtered. A planarization layer260 is formed over the entire surface of the semiconductor substrate 200including the color filter layer 250 to be disposed between the colorfilters of the color filter layer and atop a portion of the additionalstructural layer 240 that is left exposed after the photolithography ofthe respective coatings of the colored resist patterns that make up thecolor filter layer.

As shown in FIG. 2D, a material layer for microlens formation is coatedon the planarization layer 260 and it is then patterned byphotolithography to form a microlens pattern (not shown). The microlensmaterial layer may be formed as a resist layer or an oxide layer of, forexample, tetra-ethyl-ortho-silicate (TEOS). The microlens pattern isreflowed, using a hot plate or a furnace, to form a plurality ofmicrolenses 270 having a predetermined convex curvature that can bevaried according to the heat-shrinking method applied. The microlenses270 are hardened by irradiation with ultraviolet light to therebymaintain the curvature obtained by the reflowing process. Thethus-obtained structure of the microlenses 270, including theircurvature and height, determines light-concentration efficiency and thepoint of focus.

A CMOS image sensor according to the present invention is shown in FIG.2D. The plurality of photodiodes 210 are arranged at fixed intervals inthe semiconductor substrate 200, and the light-shielding layer 220blocking light arriving between the photodiodes while allowing lightsignal reception at each photodiode is arranged between the photodiodesand partially overlaps adjacent edges of the photodiodes. The insulatinginterlayer 230 is formed on an entire surface of the semiconductorsubstrate 200 including the photodiodes 210. The color filter layer 250,having a plurality of color filters separated by a predetermined gap, isformed on the insulating interlayer 230. The planarization layer 260 isformed over the entire surface of the semiconductor substrate 200including the color filter layer 250, and the plurality of microlenses270 is formed on the planarization layer in correspondence with therespective color filters. The additional structural layer 240 isdisposed between the color filter layer 250 and the insulatinginterlayer 230, to have a pattern arrangement corresponding to thejunctions of the individual color filters, and is thus provided to closethe predetermined gap between the color filters of the color filterlayer.

In the CMOS image sensor according to the present invention, it ispossible to block light incident to the gap existing between elements ofa color filter array by including, in conjunction with the color filterlayer, an additional structural layer disposed between the color filterlayer and an insulating interlayer provided under the color filterlayer. In doing so, the gap is closed and the a degradation in theimage-generating characteristics of a photodiode array of the imagesensor is prevented. The image characteristics are improved by providinggreater color separation (delineation) while approximating a colorfilter array having no gaps between individual color elements.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention covers such modifications andvariations provided they come within the scope of the appended claimsand their equivalents.

1. A method for fabricating a CMOS image sensor, comprising: forming aplurality of photodiodes at fixed intervals in a surface of asemiconductor substrate; forming a light-shielding layer on portions ofthe semiconductor substrate exposed between the photodiodes; forming aninsulating interlayer over an entire surface of the semiconductorsubstrate including the light shielding layer; forming a color filterlayer having a plurality of color filters separated by a predeterminedgap on the insulating layer; and forming an additional structural layerhaving a pattern for blocking light incident to said color filter layerat the predetermined gap on the insulating layer.
 2. The method of claim1, said additional structural layer forming step comprising: coating ablack photoresist over the entire surface of the semiconductor substrateincluding the color filter layer; and selectively patterning the coatedblack photoresist according to the predetermined gap of the color filterlayer.
 3. The method of claim 1, further comprising: forming aplanarization layer over the entire surface of the semiconductorsubstrate including the color filter layer; and forming a plurality ofmicrolenses on the planarization layer in correspondence with the colorfilters of said color filter layer.
 4. The method of claim 3, furthercomprising: irradiating the micro-lenses with ultraviolet light toharden the micro-lenses.
 5. The method of claim 1, wherein said colorfilter layer is formed by successive applications of a colored resistfor filtering light by wavelength, each colored resist being coated onthe structure provided under said color filter layer and beingseparately patterned to overlap adjacent portions of said additionalstructural layer.
 6. The method of claim 1, wherein the insulatinginterlayer is formed as a multi-layered structure.
 7. The method ofclaim 1, wherein said light-shielding layer is selectively patterned byphotolithography leaving opaque metal on portions of the semiconductorsubstrate exposed between the photodiodes.
 8. The method of claim 7,wherein said light-shielding layer has a pattern corresponding to anarrangement of the photodiodes.
 9. The method of claim 1, wherein thesaid additional structural layer is patterned to be arranged between thecolor filters of said color filter layer.
 10. The method of claim 1,wherein said additional structural layer is formed of black photoresistpatterned according to the predetermined gap of said color filter layer.